This whole area is absolutely fantastic for cycling: Not too strenuous so the whole family can enjoy the fun. There are masses of waymarked cycle tracks for you to easily follow.
Bring your own bikes or hire locally.
The Nantes-Brest canal is easy to follow along the towpath, or in Ploermel the old railway line, known as La Voie Verte goes for miles, providing easy cycling on tarmac through lovely countryside.